Seabed stabilization mattresses

ABSTRACT

A concrete or the like mattress for laying over a pipeline on the seabed comprises an elongate recess or tunnel (5) having an insulating material (6) located therein. The mattress is laid over a subsea pipeline so that the pipeline is received within the insulant-containing tunnel. The pipeline is thus physically protected and thermally insulated. The mattress can comprise a plurality of concrete or like members (1,2,2&#39;,3,3&#39;) articulated together.

The present invention relates to seabed mattresses for the protectionand stabilisation of seabed installations such as pipelines.

The use of mattresses, especially articulated mattresses, in offshore,coastal and marine engineering is well known for stabilisation,protection and scour prevention of pipelines, flowline umbilicals,seabed templates, steel and concrete platforms and the like. Thesemattresses are particularly useful in areas of high bottom current wherehydrodynamic forces are considerable. Thus, for example, a seabedpipeline can be covered with such a mattress so that the pipeline isstabilised by the weight of the blanket thereon and also the adjacentseabed is protected against erosion. Examples of articulated mattressesand their use are given for example in European patent specification152232A to which reference should be made for further details.Generally, these mattresses comprise concrete or similar elements joinedtogether to allow relative articulation.

Subsea stabilisation mattresses can be relatively massive, eg. a mat of5 m×2 m would weigh (in air) over 2.5 tonnes. However, even so, whenthey are laid over a seabed pipeline, for example, they can still beprone to movement during a storm or tidal surge. Whilst, in theory,greater stability could be obtained by increasing the mat weight, wehave found another way of dealing with this problem.

Further, in addition to stability in extreme storm conditions, astabilisation mattress should desirably also be capable of withstandingthe impact thereon of anchors or trawlboards travelling laterally, ordropped objects travelling largely vertically.

We have shown an improved mattress design in our PCT application GB890091 from which U.S. Pat. No. 5,052,859 issued to the present inventorand was assigned to the same assignee, whereby an articulated mattressis formed from elements characterised in that the mattress has arelatively thick region from which its thickness tapers to at least oneside edge.

A highly preferred feature of that invention is the provision of arecessed "tunnel" region in which a seabed pipeline is received and thusprotected by the mattress. The tunnel is preferably dimensioned to allowlateral movement of the pipeline (but not upward buckling) toaccommodate thermal expansion. In a further preferred feature of thatinvention, a heat insulation material can be introduced into the tunnelinside a container. The container is generally positioned within thetunnel recess prior to placement of the mattress, and the container isfilled from an external source.

We have now found, in accordance with one aspect of the presentinvention, a new way of providing heat insulant material in a recessedtunnel region of a mattress (such as those mattresses of our above PCTapplication, or others). According to the present invention, the heatinsulant is provided in the tunnel prior to deployment of the mattress,eg. it is provided during manufacture of the mattress or at some stageprior to emplacement of the mattress on a seabed pipeline or the like.

In one aspect, therefore, the invention provides a seabed stabilisationmattress which comprises at least one concrete or like member includinga tunnel-like recess to receive a subsea pipeline or the like, whereinthe mattress includes a heat-insulation material in the tunnel toinsulate the said pipeline or the like when the mattress is laidthereover. Preferably, the mattress comprises a plurality of concrete orlike members articulated together.

In a highly preferred embodiment of the invention, the insulant materialis such that, when the mattress has been laid over a pipeline (or thelike), it will soften or liquefy as it warms to the pipelinetemperature. Materials such as waxes or bitumen are very suitable inthat they will soften or liquefy close to the wall of the pipeline as itheats up due to the passage of contained hot fluids. This softening orliquefaction will minimise lateral resistance to any movement of thepipeline in the tunnel and allow the pipeline to take up a normallateral expansion profile without the possibilities of catastrophicupheaval buckling and failure. The insulant material will reform withinthe recess after the lateral movement has taken place and the pipelinehas changed location within the recess thus reinstating the thermalinsulation. Materials other than waxes and bitumen may be used providedthey have suitable properties. For example, certain thermoplastic resinsare useful.

The insulation material need not be one which softens or liquefies inuse. Instead, other materials may be used such as foams or othersubstances which preferably have some resilience to conform to the shapeof the pipeline surface and so provide an insulating covering thereon.Non-resilient materials can be used but they are not generallypreferred.

Additional insulation materials can be used to enhance the naturalinsulation of the concrete or other material making up the main elementsof the mattress. Materials suitable for long term seawater environment,such as polyurethane with added glass microspheres, can be used.

Insulation values equivalent to those provided by the existingtechniques, such as factory installed insulation coatings on pipelines,and soil or rock backfill around a pipeline can be achieved with thepresent invention.

The presence of the insulant material within the tunnel recess providesa number of advantages in addition to those above. For example, itallows the mattress to be lowered onto the pipeline with a much reducedchance of damage to any external corrosion coat on the pipeline wall.The insulant material in the tunnel can be shaped to act as apositioning element to ensure that the mattresses are laid preciselywith respect to the pipeline. The recess would normally be central tothe mattress such that the pipeline axis lies initially along the centreline of a row of adjacent mattresses.

A further preferred feature of the invention is that adjacent mattressesshould interlock together to resist lateral movement.

During deployment and location of a mattress of the invention, a small"lens" of seawater will often be trapped between the mattress and theseafloor by the insulant material. In one preferred embodiment of theinvention two differing types of insulant material are used, whereby onecomponent of insulant material has a specific gravity greater thanseawater such that on heating and liquefaction, this insulant flows tofill the volume occupied by the water lens and thereby surrounding thepipeline, the displaced seawater forming in the pockets away from theimmediate vicinity of the pipeline. The removal of the seawater, whichwould normally be highly oxygenated and a necessary component of anycorrosion reaction, from close proximity of the pipeline significantlyreduces the corrosion protection requirements for the pipeline coatingand cathodic protection systems. The other component insulant would havea specific gravity less than that of seawater.

Subsea pipelines which carry hot materials inevitably are subject tothermal expansion and contraction as temperatures vary. A major featureof the tunnel mattresses described in our above-mentioned PCTapplication GB 8900931 and herein is that the pipelines so protected canmove laterally but are prevented from vertical buckling. In accordancewith another aspect of the present invention, we have devised a newdesign of mattress which is particularly useful for protecting pipelinesin a trench and for avoiding upheaval buckling failure.

In accordance with this aspect of the invention, the subsea mattress isof concrete or the like, and is in section of generally triangularshape, with the apex shaped to seat on a pipeline in a trench, with thesides of the mattress engaging the sides of the trench. Preferably, themattress will comprise a plurality of members articulated together.Preferably, the mattress is of generally isosceles obtuse shape.

The trench provides the primary stabilisation of the pipeline againsthydrodynamic forces induced from seabed currents and surface waveaction. The trench also provides protection from trawlboard strike bylowering the pipeline below the seabed. Typical trenches areapproximately 1 m deep and have a slope angle of 30°. The evacuated soilgenerally forms an additional spoil pile at the extreme edges of thetrench, further enhancing the apparent depth.

Pipelines subject to considerable thermal expansion when placed in suchtrenches are subject to upheaval buckling, dependent on a number ofcritical parameters such as pressure and temperature of operation,overall length of line, local topography and presence of soil or rockdump backfill. Significant upheaval amplitudes have been experienced,including one actual failure. The present invention overcomes thisproblem by pinning the pipeline at discrete locations such that amattress of this design or a number of such mattresses are placed atprescribed intervals along the pipeline.

The action of these individual mattresses or groups of mattresses is to"pin" the pipeline within the trench such that the frictional resistancecreated between the pipeline and the seabed inhibits axial movement ofthe pipeline at the mattress location. There is then no possibility offeedthrough of the thermal expansion from one element of the pipeline toanother, this feed-through being a necessary causal mechanism of theupheaval buckling failure. The pipeline between each mattress locationis allowed to move freely within the trench and will move up the trenchwall by way of small scale local buckling to relieve the effect of thethermal expansion. The frequency of the mattress grouping will depend onsuch factors as the seabed friction co-efficient, trench geometry andpipeline temperature.

The shape of the apex of the triangular structure is designed to engagethe pipeline along the whole mattress length so that the mattress loadis translated to the pipeline evenly without local overstressing of thepipeline.

In order that the invention can be more fully understood, variousembodiments thereof will now be described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is a schematic end view of a first embodiment of the mattress ofthe invention.

FIG. 2 is a top plan view of the mattress of FIG. 1.

FIG. 3 shows a schematic view of the mattress of FIG. 1 being deployedover a seabed pipeline.

FIG. 4 shows a schematic view of the mattress of FIG. 1 in position overthe seabed pipeline.

FIG. 5 shows, on a much reduced scale, a side view of a seabed pipelinewith a continuous row of mattresses of FIG. 1 deployed over it showingthe interlocking of the mattresses.

FIG. 6 shows a schematic view of the mattress shown in FIG. 4 after hightemperature operation and some deflection of the pipeline.

FIG. 7 shows a second embodiment of the mattress of the invention.

FIG. 8 shows a schematic view of the mattress shown in FIG. 6 duringoperation.

FIG. 9 shows a middle sectional view through a mattress of FIG. 6.

FIG. 10 is a schematic view of the mattress of FIG. 4 showing a furtherpreferred feature.

FIG. 11 shows a third embodiment of the mattress of the invention.

FIG. 12 shows a bottom plan view of the mattress shown in FIG. 11.

FIG. 13 shows a schematic view of the mattress of FIG. 11 deployed ontoa seabed pipeline within a trench.

FIG. 14 shows, on a much reduced scale, a side view of the seabedpipeline and trench showing mattresses of the invention deployed atdiscrete intervals.

FIG. 15 shows the mattress of FIG. 11 in the folded condition fortransportation.

Referring to the drawings, the mattress of FIG. 1 comprises fiveelongate concrete members 1,2,2',3 and 3' lying side by side. Thecentral member 1 is the thickest and the thickness tapers down viamembers 2/2' to the side member 3/3'. Ropes or cables 4 are embedded inthe members to link them together and provide lifting or articulationpoints as described in applicant's U.S. Pat. No. 5,052,859. The centralelement 1 has a tunnel recess 5 extending the whole mattress length.Within this tunnel recess, a shaped block of insulant material 6 isformed during manufacture of the mattress. The block has a centralrecess 7 which provides the initial location for a seabed pipeline. Thecentral element 1 has a projecting section 31 which allows mattresselements to interlock.

FIG. 2 is a top plan view of the mattress of FIG. 1 and like numeralsindicate like parts. A typical size of this mattress might be width 2 m,length 5 m and maximum thickness 0.5 m. The plan view shows the insertslot 32 which provides the interlocking location for joining themattresses into rows.

FIG. 3 shows a mattress of FIG. 1 being deployed over a seabed pipeline8 lying on the seabed 9, showing the insulant recess 7 acting as a guideand protecting the pipeline from contact with concrete elements.

FIG. 4 shows the mattress of FIG. 1 in position over the seabed pipeline8 such that the seafloor 9 and the insulant recess 7 trap a discretelens of seawater 10.

FIG. 5 shows a row of mattresses of FIG. 1 on the seabed 9 over apipeline 8. The interlocking male features 31 of the central element 1are engaged with female features 32.

FIG. 6 shows the mattress of FIG. 1, placed as in FIG. 4, after thepipeline has commenced operation. The action of the passage of hightemperature fluids, typically 100° C., will cause the pipeline to expandaxially and to seek to buckle laterally. The heat generated by the hotfluid will locally melt the insulant material such that the pipeline 8can move virtually unrestrained to the position shown in FIG. 6.

The insulant has a specific gravity greater than seawater and thus theliquifying and solidifying cycle will cause the insulant to slump to thebottom of the tunnel 11.

The lens of seawater 12 migrates away from the pipeline to take up aposition such as shown enhancing the overall insulation effect of thesystem. This eliminates the oxygenated seawater from the vicinity of thehigh temperature pipeline.

FIG. 7 shows a second embodiment of the invention whereby the centraltunnel 5 has a number of insulant materials pre-installed therein priorto deployment. The immediate wall of the tunnel has a thickness ofpermanent insulant material 33 and the internal insulant is formedessentially as shown in FIG. 1 but with differing insulant materialswhere the upper insulant 34 has a specific gravity less than seawater(which is approximately 1.025) for example, a wax and the lower material35 has a specific gravity greater, than seawater, for example, abitumen-based mastic.

FIG. 8 shows the mattress of FIG. 7 placed largely as shown in FIG. 4,after the pipeline has commenced operation. The action of the hightemperature fluid as shown in FIG. 6 causes the lens of seawaterentrapped to migrate to locations 36 and 37 and the insulant 38 to slipand form around the pipeline 8.

FIG. 9 shows a section through the central element of the mattress shownin FIG. 7. Insulant elements 39, whose specific gravity is greater than1.025, are located at each end of the mattress so that the lens of wateris trapped within the confines of a mattress element to avoid migrationto adjacent mattress elements.

FIG. 10 shows a mattress of FIG. 4 where one or more inlet channels 40are provided through the central section 1 and the insulant material 6such that further insulant material can be pumped through the channel tofill the water lens 10. A vent channel 41 is provided to allow theseawater to be displaced from the area of the water lens by theadmission of this additional insulant.

FIG. 11 shows a third embodiment of the mattress of the invention. Themattress consists of a structure having an inverted triangular shapedefined by downwardly converging lower sides terminating at the apex ofan obtuse angle, the apex and at least those portions of the lower sidesadjacent the apex being in a unitary central element 21. The centralelement 21 has two outer elements, one on each side 22 and 23 and theapex is recessed at 24. The mattress is interconnected by rope linkage25 which passes freely through the central element 21 and permitsarticulation of the elements 21, 22, 23.

FIG. 12 shows a bottom plan of the mattress of FIG. 11, and likenumerals indicate like parts.

FIG. 13 shows the mattress of FIG. 11 in position in a trench 26 cutinto the seabed 9. Typical slope angle for the trench is 301 and thedepth is 1.0-1.5 metres. A spoil bank 27 often extends the side walls ofthe trench. The central element 21 lies in the centre of the trench withthe recess 24 engaging the upper surface of the pipeline 8. Theattachment rope 25 is released after deployment so that the outerelements 22 and 23 fall onto the trench side thereby supporting thecentral element such that all the load induced by the central elementbears onto the pipeline thereby maximising the axial friction force.

FIG. 14 shows a section of the trench with one or more such mattressesof FIG. 11 deployed at a regular interval along the pipeline 8. Thethermal expansion force A generated by the hot fluid in the pipelinewill be counteracted by the axial friction force B generated by thegroup of mattresses 28. The gap 29 between the groups and the size andnumber of mattresses required will depend on pipeline operationalparameters.

FIG. 15 shows the mattress of FIG. 11 but in a folded condition fortransport. It will be observed that the outer members 22 and 23 havebeen folded under the central element 21 thus making a substantiallycompact rectangular profiled unit which facilitates stacking fortransport and storage, the linking rope 25 remaining slack in the foldedconfiguration.

I claim:
 1. An articulated mattress for laying on a seabed to cover andinsulate a pipeline, said mattress comprising a plurality of elongateconcrete members which are articulated together for lying on a seabed inside-by-side and generally parallel relation, said mattress including acentral member comprising a unitary rigid concrete block having firstand second opposed side walls, and a generally planar undersurface forengagement with the seabed, an elongate tunnel in said undersurfaceextending lengthwise of said central member between said side walls, anda first insulate material installed in said tunnel prior to deploymentof said mattress over a pipeline, said insulant being constructed andarranged to cover and insulate a previously laid pipeline as saidmattress is deployed over said pipeline.
 2. A mattress according toclaim 1, wherein said first insulant material is such that, when saidmattress and insulant have been laid over said pipeline, said insulantwill soften or liquefy as it warms in response to heated fluid flowingin said pipeline.
 3. A mattress according to claim 2, wherein said firstinsulant material is selected from the group consisting of wax,bituminous material, and a thermoplastic resin.
 4. A mattress accordingto claim 1 or 2, wherein said first insulant material has a specificgravity greater than that of seawater.
 5. A mattress according to claim4, additionally comprising a second insulant material within saidtunnel, the second insulant material having a specific gravity less thanthat of seawater.
 6. A seabed stabilization mattress comprising astructure of concrete or the like which in cross section is of generallytriangular shape having downwardly converging lower sides terminating inan apex of an obtuse angle, at least those portions of said sidesadjacent said apex being parts of a unitary element of said structure,said apex being recessed to seat on a pipeline in a trench with saidsides of said structure engaging, at least in part, the sides of saidtrench.
 7. A mattress according to claim 6, wherein said recess isshaped to engage the pipeline along the whole mattress length.
 8. Amattress according to claim 6 or 7, wherein said structure comprises aplurality of elements of concrete or the like articulated together.
 9. Amattress according to claim 8, wherein said structure is of generallyisosceles shape, and wherein said elements comprise a central elementand two outer elements, each of the outer elements having a right-angledtriangular cross-section with its hypotenuse defining a part of adownwardly converging lower side of said structure.